if (empty($_POST["username"])) { $usernameError = "Username is required"; } else { $username = test_input($_POST["username"]); }This code checks if the username input is empty using the $_POST global variable. If it is empty, it sets an error message in the $usernameError variable. Otherwise, it uses the test_input() function to sanitize and validate the input data. Package/library: The PHP Form textInput is a core part of the PHP language and is therefore included in the PHP package/library. However, you can use third-party libraries such as Bootstrap or jQuery UI to enhance the styling and functionality of the form input fields.